Proteins such as fibronectin, whose function is to facilitate cell adhesion to extracellular substrate surfaces or to other appropriate cells, are synthesized by cells of most vertebrate species. Assays are available to quantitate these proteins by measuring the percentage of cells which attach and spread on the plastic surface of a tissue culture well. To study the mechanism of cell adhesion Yamada has obtained certain fibronectin-derived peptides and has ranked them in order of their effect upon fibronectin mediated BHK cell adhesion. To investigate what structural feature(s) the active peptides have in common, I have continued work with molecular models to construct some of these peptides, using literature values of bond lengths and angles. Although it was at first felt that essentially complete geometric parameters were available, it soon became apparent that enough variability in conformation still existed to preclude any conclusions concerning important structural features involved in cell binding and adhesion. Another approach to this problem was to determine the percentage of aggregation at any concentration for each peptide, in order to fit the activity level to concentration of active species. The degree of aggregation was measured by sedimentation equilibrium experiments in the analytical ultracentrifuge. A number of such measurements were made and percent dimer determined for each peptide. These data will be included in any models of the active peptides. Such models should help us to determine the crucial structural features involved in cell binding and adhesion.